﻿KILN DRYING HANDBOOK. 29 



before drying commences, it is good practice to give it a preliminary 

 steaming treatment. This is customarily continued for from \\ to 2 

 hours for each inch of thickness, the temperature being about 15° 

 above that at which the drying is to begin. The humidity should 

 be kept at 100 per cent not only during the steaming but also during 

 the subsequent cooling to the initial drying temperature. When 

 the center of the stock is already in tension a high humidity treat- 

 ment should be used instead. 



RELIEF OF INTERNAL TENSION. 



Assume that the stock has safely passed the first stages of drying 

 and that the tension in the surface has passed its maximum and is now 

 diminishing. During this period the stock is not usually liable to 

 injury, and the only surface phenomenon is the probable closing up 

 of any checks which may be in it. As soon as these checks have closed, 

 or possibly even before, the tension in the surface will have disap- 

 peared and compression begun to develop. This compression will be 

 accompanied by a corresponding tension in the core. If surface checks 

 were originally present and have closed up, the increasing tension is 

 apt to deepen them into honeycomb. If no surface checks were pres- 

 ent, the stock can stand more of the internal tension which accom- 

 panies casehardening than it could otherwise. In any event, it is 

 necessar}'- to remedy the condition or relieve the stresses before they 

 have reached a dangerous intensity. This is accomplished by soften- 

 ing the surface so that it will yield to the pull or tension of the core, 

 thus allowing the whole piece to adjust itself by shrinkage. The 

 amount of shrinkage is usually readily noticeable. The usual method 

 of softening the surface is to subject the kiln charge to a steaming 

 treatment at saturation or to a high humidity treatment at less than 

 saturation. When the depth to which the layers in compression ex- 

 tend is small — that is, when the compression " zone " is shallow — 

 steaming treatments at 100 per cent humidity are safe and satisfac- 

 tory. They will produce a quick effect on the surface, which is desir- 

 able, moistening and softening only a shallow zone, which is then 

 compressed or squeezed together by the tension or pull of the core. 

 At the time the steaming is completed the surface will still be in 

 compression and the core in tension, the amount of these stresses 

 being, however, comparatively small and representing only the force 

 required to squeeze together the wood in the compression zone, in its 

 moistened and softened condition. Immediately after the steaming 

 treatment, however, the surface layers will lose most of the moisture 

 picked up during the treatment and will shrink accordingly, thus 

 reducing all stresses and possibly even reversing them, putting the 

 surface back into tension and preparing the stock for further shrink- 

 age of the core. 



PREVENTION OF REVERSE CASEHARDENING. 



When the stock is reasonably dry and the compression zone com- 

 paratively deep, a steaming treatment may readily result in too 

 severe an effect on the surface without enough effect toward the inner 

 portion of the compression zone. If the treatment be continued long 



